1. Field of the Invention
The invention relates to a method for separating cyclic siloxanes from short-chain siloxanes having terminal hydroxyl groups and to short-chain siloxane having a low content of cyclic siloxanes.
2. Description of the Related Art
Siloxanes having terminal hydroxyl groups, for example polydimethyl siloxanes having terminal hydroxyl groups, polydimethyl(methylvinyl)siloxanes having terminal hydroxyl groups and polydimethyl(methylphenyl)siloxanes having terminal hydroxyl groups are used as crosslinking agents, plasticizers and intermediates in the silicone industry. These siloxanes are produced, for example, by hydrolysis of dimethyldichlorosilane, methylvinyldichlorosilane or methylphenyldichlorosilane in the presence of chlorosiloxanes or of alkoxysiloxanes. The resultant product is separated from water in order to prepare a hydrolyzate mixture which, in addition to the desired short-chain siloxanes having terminal hydroxyl groups, also contains cyclic siloxanes.
The cyclic siloxanes D4 (octamethylcyclotetrasiloxane), D5 (decamethylcyclopentasiloxane) and D6 (dodecamethylcyclohexasiloxane) are listed by the OECD as “high production volume chemicals” (>1000 t/year in at least one OECD member). Owing to the great production volume thereof and the increasing use which results in an increased exposure of humans and increased introduction into the environment, siloxanes have been studied with respect to ecotoxicological criteria such as environmental persistence and bioaccumulation, and also with respect to toxic effects thereof to humans and animals. Siloxanes fulfill some of the preconditions necessary for classification as PBT substances (“persistence, bioaccumulation, toxicity”). They are relatively stable, can be transported over wide distances in the atmosphere and have a sufficient physical and biological half life in order to accumulate in the environment and in aquatic creatures.
From this aspect there is a need to provide short-chain siloxanes having terminal hydroxyl groups having a low content of cyclic siloxanes, and a method for production thereof.
Methods for producing linear siloxanes and separating them from volatile compounds, inter alia from cyclic siloxanes, are known (see methods A-D).
(Method A)
WO 9410228 describes a method for purifying liquid polysiloxane which contains impurities as a low-molecular-weight fraction by vaporizing the low-molecular-weight fraction. Here, owing to the low boiling point, the low-molecular-weight fractions of the short-chain siloxane are also separated conjointly (see in this context the present comparative example) and the chain length distribution of a short-chain siloxane having terminal hydroxyl groups is changed. The consequence thereof is a changed property profile such as, for example, a higher viscosity, etc., or differences in the application properties, e.g. as plasticizer.
(Method B)
U.S. Pat. No. 4,096,160 describes separating low-boiling cyclic polysiloxanes from linear hydroxyl-terminal diorganopolysiloxane liquid using steam under a partial vacuum of at most 70 mm Hg. In this process, residual cyclene contents in the linear hydroxyl-terminal diorganopolysiloxane of <2% are achieved. However, on account of the low boiling point, the low-molecular-weight fractions of the short-chain siloxane are also separated conjointly and the chain-length distribution of the desired short-chain siloxane having terminal hydroxyl groups is changed thereby.
(Method C)
EP 1013698 describes a method for the continuous reduction of the amount of cyclic organosiloxanes in a circulated process stream in which a low-boiling fraction containing low-molecular-weight cyclic organosiloxanes and an inert solvent is separated from the process stream by distillation. In this process, as in method A, the low-molecular-weight fractions of a short-chain siloxane are also separated conjointly.
(Method D)
EP 543665 claims a purification method for long-chain siloxanes by contacting the siloxane with steam and distilling out and separating the impurities with the steam. The impurities comprise cyclic and short-chain siloxanes.